import re
pat = r"([^\W\d_])([^\W\d_]*)([^\W\d_])"
s = "T3sters"
print(re.sub(pat, (lambda m: "{0}{1}{2}".format(m.group(1), len(''.join(set(m.group(2)))), m.group(3))), s))
aW1wb3J0IHJlCnBhdCA9IHIiKFteXFdcZF9dKShbXlxXXGRfXSopKFteXFdcZF9dKSIKcyA9ICJUM3N0ZXJzIgpwcmludChyZS5zdWIocGF0LCAobGFtYmRhIG06ICJ7MH17MX17Mn0iLmZvcm1hdChtLmdyb3VwKDEpLCBsZW4oJycuam9pbihzZXQobS5ncm91cCgyKSkpKSwgbS5ncm91cCgzKSkpLCBzKSk=